Manganese-molybdenum ductile steel



NOV. 1954 HAJIME NAKAMURA 3,

MANGANESE-MOLYBDENUM DUCTILE STEEL Filed April 20, 1962 5 Sheets-Sheet 1 GI? 930C xl hour, Woler- Cooled;

650Cx l.5 hours, Air-Cooled N.T.; 930Cxl hour, Air- Cooled;

650Cxl5 hours,Air- Cooled Precipitated Aluminum Ni1ride,%

INVENTOR. HAJIME NAKAMURA ATTORNEY 1964 HAJIME NAKAMURA 3,155,496

MANGANESE-'MOLYBDENUM DUCTILE STEEL Filed April 20, 1962 s Sheets-Sheet 2 0.1. 930C x |hour,W.

650Cx L5 hours,AC.

NI 1930Cx I hour AC;

650Cxl.5 hours,AC.

Dissolved Metallic Aluminum in Solid Solution, A

INVENTOR HAJIME NAKAMURA ATTORNEY 1964 HAJlME NAKAMURA 3,155,496

MANGANESE-MOLYBDENUM DUCTILE STEEL Filed April 20, 1962 5 Sheets-Sheet s Tuz. E.

01.1 Woter-Quenchand-Tempered ommercial Steel,Q.T.

I I s Commerciol$teel,N.T. l I

-200480460-l40-l20-lOO-80-60-40-2O 0 2O 40 6O 80 Temperuture,C

INVENTOR. HAJIME NAKAMURA BY W ATTORNEY 1954 HAJIME NAKAMURA 3,155,496

MANGANESE-MOLYBDENUM DUCTILE STEEL Filed April 20, 1962 5 Sheets-Sheet 4 Hg. I; Specimen Ken-3, 930C at 1 hr Air-Cooled Fig. 5 Specimen X10. 3, 930C 1: 1 hr Air-Cooled x 10000 INVENTOR.

Ha ime Nakam ra BY We g Mia MOmeys Nov. 3, 1964 HAJlME NAKAMURA 3,155,496

MANGANESE-MOLYBDENUM DUCTILE STEEL Filed April 20, 1962 s Sheets-Sheet s Fir. 6 Specimen No. 3, 930C x 1 hr dawn-Cooled; 650C 1: 1-5 hrs Air-Cooled Fig. 7 Specimen No. 3, 930C x 1 hr Water-Cooled:

650C 2: 1.5 hrs Air-Cooled INVENTOR H me Naka mum Mtg & mu At neys United States Patent 2 Claims. (61. 75-124 Although the so-called manganese-molybdenum grade steels have been used extensively as the specified material for boilers, like for example, as the A-302 Grade B according to the ASTM Standard, recently it has been recognized that such steels are also suited for nuclear reactor pressure vessels, as its neutron absorption cross section is relatively suitable.

However, for the Mn-Mo steels to be employed in construction of a nuclear reactor pressure vessel, the current specifications have it that the Charpy V-notch impact value be over 30 ft.-lbs. (5.2 kg.-m./cm. at 12 C. in order to counter the embrittling effect of radiations. This is to say, that excellent properties are being demanded of a Mn-Mo steel both at low temperatures and at elevated temperatures. As may be readily imaginable, such requirement as mentioned above is one that is very diflicult to achieve. Thus, it is not without reason, that steel manufacturers are running into difficulties in producing Mn-Mo grade steel for nuclear reactor services.

The present inventor has already discovered that an excellent ductility may be realized in a steel without spoiling, rather in effect improving, its general mechanical properties by prescribing the amount of precipitated aluminum nitride and metallic aluminum dissolved in solid solution in carbon and low-alloy steels. Namely, the low temperature ductility of said steels are remarkably improved, and the high temperature properties thereof, such as the creep characteristics, are at least equal, if not actually superior, to commercially available steels with equivalent chemical composition save the aluminum nitride content. The present inventor then was led to a conclusion that there is a certain limit for aluminum nitride in precipitation and metallic aluminum in solid solution to be effective.

The present invention has accomplished to provide an improved Mn-Mo grade steel for uses in boiler drum as well as nuclear reactor vessel and other applications. Namely, the present invention relates to a so-called manganese-molybdenum grade steel containing less than about 0.25% carbon, less than about 0.60% silicon, about 1.00 to 2.00% maganese, about 0.30 to 1.00% molybdenum, less than about 0.030% phosphorus, less than about 0.040% sulphur, and as an optional additive element, less than about 1.00% chromium, featuring an amount fo aluminum nitride as precipitated by about 0.006 to 0.10% and an amount of metallic aluminum dissolved in the matrix in solid solution by less than about 0.15%, as well as a refined granular structure in which the ferrite grain size is over No. 9 in terms of the ASTM ferrite grain size number system. The present invention also relates to said steel except in a heat treated condition comprising either heating thereof at a temperature above the transformation point and within a range of maximum precipitation of said aluminum nitride followed by cooling the same steel, or heating thereof at a similar temperature as above and subsequently reheating or tempering the same at a low temperature followed by cooling.

In what follows the principle and the scope of the present invention will be disclosed in full detail, in which references will be made to drawings and photograms, where,

ice

FIG. 1 is a drawing to show the relation between various transition temperatures of a Mn-Mo grade steel and the amount of precipitated aluminum nitride there- FIG. 2 is a drawing to show the relation between various transition temperatures of a Mn-Mo grade steel and the amount of metallic aluminum dissolved therein in solid solution,

FIG. 3 is a drawing to compare the transition temperature characteristics of a Mn-Mo grade steel according to the present invention to that of a like steel which is available on existing commercial market,

FIGS. 4 and 6 are electron photomicrograms to illustrate the granular structure of a Mn-Mo grade steel according to the present invention in a state of normalization at magnifications of 4,000 and 10,000 respectively,

FIGS. 5 and 7 are electron photomicrograms to illustrate the granular structure of the same steel as above except in a state of quench-and-tempering at magnifications of 4,000 and 10,000 respectively.

The chemical composition representative of steels that were employed in various tests is summarized in Table 1. It is to be seen therein that those steels are all of typical composition as Mn-Mo grade steel, except the N and AJN contents.

The relation between the amount of precipitated aluminum nitride versus the 15 ft.-lbs. transition temperature, Tr15, and the 30 ft.-1bs. transition temperature, Tr30, and the fracture transition temperature, TrS, is as shown in FIG. 1 for a Mn-Mo grade steel of a tensile strength about 70 kg./mm. level in two different states of normalize-and-tempering and quench-and-tempering. It will be seen that, in order to obtain an impact value more than 30 ft.-lbs. at l2 C., the minimum amount of precipitated aluminum nitride is about 0.006% and the amount of precipitated aluminum nitride corresponding to the lowest transition temperature is around 0.04 to 0.05% for normalize-and-tempered material, and 0.05 to 0.06% for quench-and-tempered stock. It will be further noted that the transition temperature is raised as the amount of precipitated aluminum nitride is increased beyond this limit as mentioned above.

FIG. 2 shows the change of various transition temperatures when the amount of metallic aluminum dissolved in solid solution in a Mn-Mo grade steel is varied for a given amount of precipitated aluminum nitride of 0.075 to 0.080% level. It is to be recognized that the transition temperature suffers hardly any change within the temperature range used in experiments for the amounts of metallic aluminum in solid solution increased beyond 0.05 It is also to be seen that, with as much metallic aluminum in solid solution as 0.12%, an excellent property may be obtained in that the Tr30 is 54 C. in a state of normalize-and-tempering and C. in quench-and-tempered condition.

FIG. 3 comparatively shows the representative Charpy V-notch transition temperature curve obtained from a 17 mm. thick plate of a Mn-Mo grade ductile steel according to the present invention and that obtained from a 70 mm. thick plate of a Mn-Mo grade steel acquired from commercial market. While a certain portion of the difference exhibited by this figure as existing between those two varieties of same grade steel with respect to the transistion temperature characteristics may be attributed to the difference in the thickness of respective plates, namely 17 mm. for the steel of the present invention as against 70 mm. for the commercial stock, because such thickness effect is known to affect on the low temperature characteristics adversely, yet such superiority of the steel due to present invention over commercial steel in the field of low temperature ductility as shown by the FIG. 3 should be regarded as real and of a substantial magnitude. Such conclusion may be drawn if the natural quench-hardenability of Mn-Mo grade steels and the comparability of cooling rate of water-quenching a 70 mm. thick plate of commercial steel and that of air-cooling a 17 mm. thick plate of the present steel are both taken into consideration. Therefore, the superiority of low temperature ductility of the steel according to present invention should be attributed to the exceptional fineness of grain that is caused by the action of precipitated aluminum nitride on one hand, and the suppression of grain coarsening effect of the metallic aluminum dissolved in solid solution on the other, by carefully controlling the amount of the latter.

The granular structure of a Mn-Mo steel of present invention is shown in FIGS. 4 and 5 with respect to the specimen No. 3 of Table 1 in a state of normalization, i.e., heating at 930 C. for one hour followed by cooling in air; and in FIGS. 6 and 7 with the same material but in a state of quench-and-tempering, i.e., heating at 930 C. for one hour, then cooling in water, reheating at 650 C. for 1.5 hours followed by cooling in air. It Will be seen that the ferrite grain size of this specimen is no coarser than No. 9 to 12 in either case as compared with normal commercial steels No. 5 to 7, both in terms of the ASTM ferrite grain size number.

It will further be seen on comparing FFIG. 4 to FIG. 6, and FIG. 5 to FIG. 7, that the grain size is always smaller in the state of quench-and-tempering than in the state of normalization, the former corresponding to about No. 11 to 12 range, while the latter to about No. 9 to range, both in terms of ASTM ferrite grain size number.

In these photomicrograms, the dark rectangular shaped objects are the precipitated aluminum nitride, of whose refining action of crystal grains mention has already been made. Besides this effect of precipitated aluminum nitride as mentioned above, the aluminum nitride, when precipitated out finely, dispersedly and uniformly through the granular structure, as seen in aforementioned photo micrograms, works to prevent the deformation to progress unduly, and particularly helps to arrest the crack to propagate. In other words, the steel due to the present invention is evidently capable of withstanding at least an equal, if not actually larger, magnitude of stress in creep deformation at an elevated temperature as compared with commercially available same grade steel, and furthermore, the elongation at rupture is improved over normal commercial steel of equivalent chemical composition. Another reason for this improved high temperature ductility associated with the steel of present invention may be sought in a fact that the grain size thereof is very uniform as may be judged from aforementioned photomicrograms, as it has been known in the art that nonuniformity of grain size, or the mixed granular structure, is unfavorable for the mechanical properties.

Thus, the Mn-Mo grade steel according to present invention may be obtained with excellent mechanical properties in states of as-rolled, as-forged or like, but further refined granular structure and yet improved properties associated with such structure are to be realized when a process of heat treatment involving rapidly cooling thereof from a temperature above the transformation point and within the range of maximum precipitation of aluminium nitride, followed by, if desired, tempering at a low temperature.

Although no chromium component was involved in the specimens used in experiments, as well known in the art that the addition of chromium is effective to suppress excessive graphitization of a steel containing a large amount of aluminum, and to prevent the crack formation due to such graphitization when put to services at a temperature beyond 400 C., such chromium addition by an amount about 0.3 to 1.0% may also be needed for the steel of present invention, particularly when services at a temperature over 400 C. are expected.

Though only 1.2% Mn-0.5% Mo class steels were mentioned in the foregoing paragraphs, it is merely for the description and explanations sake, and it is not to be regarded as limiting the scope of the present invention except by amended claims, as follows.

I claim:

1. A manganese-moylbdenum low-alloy steel consisting essentially of, by weight, 1.00% to 2.00% manganese, 0.30% to 1.00% molybdenum, less than 0.25% carbon, less than 0.60% silicon, less than 0.030% phosphorus, less than 0.040% sulphur, 0.006% to 0.10% precipitated aluminum nitride, less than 0.15% metallic aluminum dissolved in solid solution, the balance iron and incidental impurities, said steel having a fine ferrite granular structure that is finer than number 9 in terms of the ASTM ferrite grain size numbering system.

2. A manganese-molybdenum low-alloy steel consisting essentially of, by weight, 1.00% to 2.00% manganese, 0.30% to 1.00% molybdenum, less than 1.00% chromium, less than 0.25% carbon, less than 0.60% silicon, less than 0.030% phosphorus, less than 0.040% sulphur, 0.006% to 0.10% precipitated aluminum nitride, less than 0.15% metallic aluminum dissolved in solid solution, the balance iron and incidental impurities, said steel having a fine ferrite granular structure that is finer than number 9 in terms of the ASTM ferrite grain size numbering system.

References Cited in the file of this patent UNITED STATES PATENTS 2,229,140 Smith Jan. 21, 1941 2,253,812 Rooke et al. Aug. 26, 1941 2,797,162 Korczynsky June 25, 1957 2,829,996 Murphy Apr. 8, 1958 3,009,844 Connert Nov. 21, 1961 FOREIGN PATENTS 515,709 Belgium Dec. 15, 1952 

1. A MANGANESE-MOYLBDENUM LOW-ALLOY STEEL CONSISTING ESSENTIALLY OF, BY WEIGHT, 1.00% TO 2.00% MANGANESE, 0.30% TO 1.00% MOLYBDENUM, LESS THAN 0.25% CARBON, LESS THAN 0.60% SILICON, LESS THAN 0.030% PHOSPHORUS, LESS THAN 0.040% SULPHUR, 0.006% TO 0.10% PRECIPITATED ALUMINUM NITRIDE, LESS THAN 0.15% METALLIC ALUMINUM DISSOLVED IN SOLID SOLUTION, THE BALANCE IRON AND INCIDENTAL IMPURITIES,SAID STEEL HAVING A FINE FERRITE GRANULAR STRUCTURE THAT IS FINER THAN NUMBER 9 IN TERMS OF THE ASTM FERRITE GRAIN SIZE NUMBERING SYSTEM. 